Power amplifiers are used in transmitters to rebroadcast, at high power levels, amplitude modulation (AM) signals, frequency modulation (FM) signals, and quadrature amplitude modulation (QAM) signals. As is known, information is carried in the amplitudes of AM and QAM signals. Because of this, transmitters must accurately rebroadcast these signals, thus the power amplifiers within the transmitters must have high fidelity, i.e. linearly rebroadcast the signals they receive.
To maintain linear performance, the power elements (usually transistors) are operated at levels much below their rated output power levels. Thus, to achieve high output power levels, many power amplifiers include cascaded elements, such that the amplified output of one element is the input of the next element. In this manner, a small change in a first stage typically produces a large change in the final stage output. For this reason, the power amplifier utilizes a feedback control loop, which is regulated by a power control circuit, to maintain output power levels. See FIG. 1 which illustrates two power elements cascaded together, wherein the power level is controlled by a power control circuit.
To key-up (transmit) or de-key (cease transmitting) a transmitter, a substantial change in the output power level of the power amplifier is required. Federal Communications Commission (FCC) regulations require transmitters to key-up and de-key very quickly while also minimizing splatter on adjacent channels. To accomplish a rapid transmitter power output level adjustment, the control loop enters into a fast mode of operation for a fixed amount of time which is typically set by a capacitor. When the capacitor becomes charged, the loop settles into a slower more stable mode of operation. For a given set of cascaded elements within a given power amplifier, this method works well at fixed temperatures and output power adjustments. However, if any of these parameters change, the same capacitance produces performance fluctuations. As a result, the transient time may be longer and the output power may overshoot to dangerously high power levels. In addition to achieving dangerously high power levels, the overshooting causes splattering on adjacent channels, i.e. interference with adjacent channels. See FIG. 2 which illustrates a prior art timing response of the relationship between the control voltage (V.sub.C) and output power (P.sub.OUT).
To help reduce the overshoot, one prior art solution was to operate the power amplifier at a higher gain level for a fixed time during an output power adjustment. The fixed time was selected to accommodate worst case conditions. Thus, when these worst case conditions were active, the overshoot was minimal. However, when the output adjustments were not worst case, the overshoot returned and the key-up time would vary based on the output adjustment. FIG. 3 illustrates the timing relationship between the control voltage (V.sub.C) and output power (P.sub.OUT). As shown, for the worst case situation, the overshoot was minimal, however the delay time, t.sub.d, was excessively long. Alternatively, when the delay time was short, the overshoot was substantial.
Therefore, a need exists for a method and apparatus that controls transient responses of a power amplifier such that the transient time is predictable for a variety of output power adjustments with minimal overshoot.